1. Field of the Invention
The present invention relates generally to a wireless mobile communication system and, more particularly, to an instruction for transition to a sleep state at the base station for conserving transmit power and operations of the base station and a terminal for supporting transition between the active and sleep states of the base station.
2. Description of the Related Art
In the field of wireless mobile communication, reducing the transmit/receive power of the base station is a frequently raised issue, and there have been efforts to reduce power consumption of the base station in a Long Term Evolution (LTE) communication system, which is one of the mostly widely utilized communication systems. One approach of reducing the power consumption of the base station is to introduce the concept of a dormant state, which the base station enters when there is no terminal connected thereto, so as to stop unnecessary transmissions of a common channel, resulting in power conservation. The base station transitions between the active and dormant states depending on the situation. The terms ‘active state’ and ‘dormant state’ may be interchanged with other terms having the same meanings In the present disclosure, the term ‘dormant state’ denotes a state in which the base station suspends transmission of the common channel such as, for example, a Common Reference Signal (CRS), and the term ‘active state’ denotes a state in which the base station is capable of transmitting/receiving all types of channels.
In LTE, the deployment of a plurality of small cells in the shadow area of a macro cell is very useful in reducing the load of the macro cell and facilitating service to a high density of mobile terminals. In such a hotspot area, the density of terminals varies significantly depending on the time of day, such that the load of the macro cell increases at a time when people gathers such as, for example, day time or evening, resulting in degradation in service quality. Accordingly, if a plurality of small cells is deployed within the macro cell, especially at that time when the density of terminals increases, the service quality to the terminals may be maintained. During times such as late night or dawn, when the number of terminals decreases in the hotspot, the macro cell may be able to serve all of the terminals within its coverage without assistance from the small cells. When the number of terminals serving within the hotspot is small, maintaining the small cells in an active state is not necessary, and thus, it is preferred that the small cells enter the dormant state to prevent unnecessary power consumption.
FIG. 1 is a diagram illustrating states of small cells within a macro cell. A macro cell 102 is under the control of a base station 101 and includes a plurality of small cells 103 to 109. In the situation illustrated in FIG. 1, the terminals are gathered in the coverage areas of the small cells 103, 104, 105, and 106, and it is advantageous for the small cells 103, 104, 105, and 106 to stay in the active state for the purpose of load balancing from the view point of the base station 101. However, it is not necessary for the small cells 107, 108, and 109, which have no terminals to serve, to stay in the active state. In the case of FIG. 1, it is advantageous to control the small cells 107, 108, and 109 to enter the dormant state from the view point of energy conservation of the base station.
If the small cells, which are deployed in a hotspot, transition between the active and dormant states depending on the number of terminals to serve and/or a data load amount of the hotspot, it is possible to conserve the power of the small cells, which is otherwise unnecessarily consumed in the conventional system.